Method and device to acquire seismic data

ABSTRACT

Streamer and method for deploying the streamer for seismic data acquisition related to a subsurface of a body of water. The method includes a step of releasing into the body of water, from a vessel, a body having a predetermined length together with plural detectors provided along the body; a step of towing the body and the plural detectors such that the plural detectors are submerged; and a step of configuring plural birds provided along the body, to float at a predetermined depth from a surface of the water such that a first portion of the body has a curved profile while being towed underwater.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/464,149 filed on May 4, 2012. U.S. patent application Ser.No. 13/464,149 is a continuation of U.S. patent application Ser. No.13/272,428 filed on Oct. 13, 2011. U.S. patent application Ser. No.13/272,428 claims the benefit of priority of U.S. ProvisionalApplication 61/392,982, having the title “Method and Device to AcquireSeismic Data,” and being authored by R. Soubaras, the entire content ofwhich is incorporated herein by reference.

BACKGROUND

1. Technical Field

Embodiments of the subject matter disclosed herein generally relate tomethods and systems and, more particularly, to mechanisms and techniquesfor acquiring seismic data.

2. Discussion of the Background

During the past years, the interest in developing new oil and gasproduction fields has dramatically increased. However, the availabilityof land-based production fields is limited. Thus, the industry has nowextended drilling to offshore locations, which appear to hold a vastamount of fossil fuel. Offshore drilling is an expensive process. Thus,those engaged in such a costly undertaking invest substantially ingeophysical surveys in order to more accurately decide where to drill inorder to avoid a dry well.

Marine seismic data acquisition and processing generate a profile(image) of the geophysical structure (subsurface) under the seafloor.While this profile does not provide an accurate location for the oil andgas, it suggests, to those trained in the field, the presence or absenceof oil and/or gas. Thus, providing a high resolution image of thesubsurface is an ongoing process for the exploration of naturalresources, including, among others, oil and/or gas.

During a seismic gathering process, as shown in FIG. 1, a vessel 10 towsan array of acoustic detectors 12. The array of acoustic detectors 12 isdisposed along a body 14. Body 14 together with its correspondingdetectors 12 are sometimes referred to, by those skilled in the art, asa streamer 16. The vessel 10 may tow plural streamers 16 at the sametime. The streamers may be disposed horizontally, i.e., lying at aconstant depth z₁ relative to a surface 18 of the ocean. Also, theplural streamers 16 may form a constant angle (i.e., the streamers maybe slanted) with respect to the surface of the ocean as disclosed inU.S. Pat. No. 4,992,992, the entire content of which is incorporatedherein by reference. FIG. 2 shows such a configuration in which all thedetectors 12 are provided along a slanted straight body 14 making aconstant angle α with a reference horizontal line 30.

With reference to FIG. 1, the vessel 10 may also tow a sound source 20configured to generate an acoustic wave 22 a. The acoustic wave 22 apropagates downward and penetrates the seafloor 24, eventually beingreflected by a reflecting structure 26 (reflector R). The reflectedacoustic wave 22 b propagates upwardly and may be detected by detector12. For simplicity, FIG. 1 shows only two paths 22 a corresponding tothe acoustic wave. However, the acoustic wave emitted by the source 20may be substantially a spherical wave, e.g., it propagates in alldirections starting from the source 20. Parts of the reflected acousticwave 22 b (primary) are recorded by the various detectors 12 (therecorded signals are called traces) while parts of the reflected wave 22c pass the detectors 12 and arrive at the water surface 18. Since theinterface between the water and air is well approximated as aquasi-perfect reflector (i.e., the water surface acts as a mirror forthe acoustic waves), the reflected wave 22 c travels back towards thedetector 12 as shown by wave 22 d in FIG. 1. Wave 22 d is normallyreferred to as a ghost wave because this wave is due to a spuriousreflection. The ghosts are also recorded by the detector 12, but with areverse polarity and a time lag relative to the primary wave 22 b. Thedegenerative effect that ghost waves have on bandwidth and resolution ofseismic measurements are known. In essence, interference between primaryand ghost arrivals causes, among other problems, notches, or gaps, inthe frequency content of the data recorded by the detectors.

The traces may be used to determine the subsurface (i.e., earthstructure below surface 24) and to determine the position and presenceof reflectors 26. However, the ghosts disturb the accuracy of the finalimage of the subsurface and for at least this reason, various methodsexist for removing the ghosts, i.e., deghosting, from the results of aseismic analysis.

The streamer configuration illustrated in FIG. 2 is considered toprovide a more accurate data acquisition then the configurationillustrated in FIG. 1. One reason for this difference is the fact thatfor each reflector, a time gap between the detection of the primary andghost reflections becomes greater, the further the detector 12 is fromthe source 20, due to the slanted disposition of the detectors, thusfacilitating deghosting.

However, the slanted streamer shown in FIG. 2 has the followinglimitation, which makes it impracticable. Current streamers have atypical length on the order of 6 to 10 km. Using a slanted streamer assuggested in U.S. Pat. No. 4,992,992, e.g., with a slope of 2 degreesrelative to the horizontal line 30, would lead to a depth of about 280 mfor the last detector, while in reality current marine detectors aredesigned to operate in water depths up to about 50 m. Thus, for currentstreamers, the approach proposed in the '992 patent would requiredetectors to be located in water depths beyond their currentcapabilities, thus resulting in detectors failure or the impossibilityto provide the detectors at those depths.

In order to accurately locate deep reflectors, high-frequency acousticwaves are not suitable on account of the high attenuation they undergoduring their propagation. Thus, low-frequency acoustic waves are desiredto be present in the spectrum recorded by the detectors. Therefore, anoctave is desirable to be gained in the low-frequency range of thetraditional methods, thereby increasing the conventional bandwidth of5-40 Hz to, e.g., a bandwidth of 2.5-40 Hz. To gain the extra octave, itis possible to increase the depth of the streamer. However, it is notsufficient to give priority to low-frequencies since high-frequenciesare needed to estimate precisely the velocity model of the surfacelayers. Also, the signal-to-noise ratio should be improved forlow-frequency acoustic waves without deteriorating it for thehigh-frequency acoustic waves.

Therefore, although a slanted streamer may partially extend theabove-discussed bandwidth due to the constant depth increase of thedetectors relative to the surface of the water, additional limitationsexist in the approach proposed by the '992 patent, as furtherillustrated below.

FIGS. 3 and 4 illustrate the simulated effect of ghosts on the frequencyspectrum (herein referred to as “effective spectrum”) corresponding to ashallow reflector (disposed at a depth of about 800 m) for a slantedstreamer after stacking (stacking is a process in which different tracescorresponding to the same common point are added together to reducenoise and improve overall data quality). In other words, spectra 34 and38 correspond to different depths of the detectors with no ghosts whilespectra 36 and 40 correspond to the same different depths of thedetectors but with ghosts. These spectrum simulations are for a streamerhaving the first detector placed at a depth relative to the surface ofthe water of about 7.5 m and about 15 m. It is noted that relativevalues of the amplitudes of the frequencies are plotted against thefrequencies in FIGS. 3 and 4. In both of the ghost free simulations (34and 38), the last detector on the slanted streamer is placed at a depthof about 37.5 m relative to the water surface. The ghost free simulatedspectra (curves 34 and 38, respectively) neglect the ghost effect, i.e.,represent an “idealized” situation in which the presence of ghosts hasbeen artificially removed from the simulation, so as to show thedeficiencies of the conventional data acquisition methods. The effectivespectra 36 and 40 are calculated without artificially removing theeffect of the ghosts. As clearly shown in FIGS. 3 and 4, the two spectra34 and 36 of the first configuration have different shapes as theeffective spectrum 36 includes less energy for the low-frequencies(lower than about 10 Hz) and high-frequencies (higher than about 60 Hz)as compared to the ghost free simulated spectrum 34. As the final imageof the subsurface is sensitive to the low- and high-frequencies, twospectra that have these portions different from each other areconsidered to be different and thus, the data corresponding to theeffective spectrum 36 does not produce an accurate final image of thesubsurface.

An advantage of increasing the depth of the first detector is tominimize the effect of swell noise, e.g., noise produced by swells atthe surface of the water. The swell noise is known to mainly affectdetectors close to the surface of the water. A simulated spectrum 38 forthis situation and a corresponding effective spectrum 40 are shown inFIG. 4. However, even for this case, the effective spectrum 40 shows anotch at about 45 to 50 Hz that corresponds to a detector having a depthof about 15 m.

For a reflector at a depth of about 15 m, the data recorded by thedetectors relatively close to the seismic source may have an overridinginfluence in stacking as the distant receivers make a less importantcontribution. Therefore, for a shallow reflector, mostly the recordingsof the detectors positioned in the head portion of the streamer (closestto the vessel) are used. This means that the depth dynamics of thedetectors, which determine the diversity of the notches, areinsufficient for good quality ghost elimination.

As can be seen from the above summarized illustrative discussion, asubstantial disparity at low- and high-frequencies of the spectrum arestill present when using streamers with a constant slant, which resultsin a poor final image of the subsurface. Accordingly, it would bedesirable to provide systems and methods that avoid or significantlyreduce the afore-described problems and drawbacks of the conventionalsystems.

SUMMARY

According to an exemplary embodiment, there is a streamer for collectingseismic data related to a subsurface of a body of water. The streamerincludes a body having a predetermined length; plural detectors providedalong the body; and plural birds provided along the body. The birds areconfigured to float, when deployed under water, at predetermined depthsfrom a surface of the water such that a first portion of the body has acurved profile while being towed underwater.

According to another exemplary embodiment, there is a method for seismicdata acquisition related to a subsurface of a body of water. The methodincludes a step of towing a body having a predetermined length togetherwith plural detectors provided along the body; a step of configuringplural birds provided along the body, to float at predetermined depthsfrom a surface of the water such that a first portion of the body has acurved profile while being towed underwater; and a step of recordingwith the plural detectors the seismic data generated in the body of thewater while towing the body.

According to still another exemplary embodiment, there is a method fordeploying a streamer for seismic data acquisition related to asubsurface of a body of water. The method includes releasing into thebody of water, from a vessel, a body having a predetermined lengthtogether with plural detectors provided along the body; towing the bodyand the plural detectors such that the plural detectors are submerged;and configuring plural birds provided along the body, to float atpredetermined depths from a surface of the water such that a firstportion of the body has a curved profile while being towed underwater.

According to still another exemplary embodiment, there is a streamer forcollecting seismic data related to a subsurface of a body of water. Thestreamer includes a body having a predetermined length; plural detectorsprovided along the body; and plural birds provided along the body. Thestreamer is configured to flow underwater when towed such that theplural detectors are distributed along a curved profile described by (i)a depth z₀ of a first detector, (ii) a slope s₀ of a first portion ofthe body with an axis parallel with a surface of the body of water, and(iii) a predetermined horizontal distance h_(c) between the firstdetector and an end of the curved profile.

According to yet another exemplary embodiment, there is a streamer forcollecting seismic data related to a subsurface of a body of water. Thestreamer includes a body having a predetermined length; plural detectorsprovided along the body; and plural birds provided along the body. Thestreamer is configured to flow underwater when towed such that theplural detectors are distributed along a first portion and a secondportion of the body. The first portion has a slanted profile with afirst slope and the second portion has a slanted profile with a secondslope different from the first slope.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate one or more embodiments and,together with the description, explain these embodiments. In thedrawings:

FIG. 1 is a schematic diagram of a conventional seismic data acquisitionsystem having a horizontal streamer;

FIG. 2 is a schematic diagram of a conventional seismic data acquisitionsystem having a slanted streamer;

FIGS. 3 and 4 illustrate conventional frequency spectra for the dataacquisition systems shown in FIG. 2 for two different depths;

FIG. 5 is a schematic diagram of a seismic data acquisition systemhaving a curved streamer according to an exemplary embodiment;

FIG. 6 is a schematic diagram of a seismic data acquisition systemhaving a curved streamer according to another exemplary embodiment;

FIG. 7 is a schematic diagram of a seismic data acquisition systemhaving a curved streamer according to still another exemplaryembodiment;

FIG. 8 is an image of a subsurface according to a conventional streamerconfiguration;

FIG. 9 is an image of the same subsurface according to a novel streamerconfiguration;

FIG. 10 is a graph showing ramp rate differences between a conventionalstreamer configuration and several novel streamer configurationsaccording to the subject matter disclosed;

FIG. 11 is a schematic diagram of a seismic data acquisition systemhaving a double slanted streamer according to an exemplary embodiment;

FIGS. 12 and 13 are graphs showing a frequency spectrum for a streamerwith multiple slants at different depths according to exemplaryembodiments;

FIG. 14 is a schematic diagram of a seismic data acquisition systemhaving a streamer with multiple slants according to an exemplaryembodiment;

FIG. 15 is a flow chart illustrating a method for seismic dataacquisition according to an exemplary embodiment; and

FIG. 16 is a flow chart illustrating a method for deploying a streameraccording to an exemplary embodiment.

DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. The following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims. The following embodimentsare discussed, for simplicity, with regard to the terminology andstructure of a streamer having a profile that varies with depth, atleast initially. However, the embodiments to be discussed next are notlimited to these structures, but may be applied to other structures thathave detectors provided at a variable depth.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with an embodiment is included inat least one embodiment of the subject matter disclosed. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout the specification is not necessarily referringto the same embodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

According to an exemplary embodiment, there is a plurality of detectorsprovided (attached or build into the streamer) on a streamer andconfigured to be distributed at variable depths, on a curved profile,relative to a horizontal line (e.g., surface of the water). It is notedthat the detectors may follow the curved profile while the streamer maynot, e.g., the actual body of the streamer to which the detectors areattached may deviate from the curved profile but an imaginary line drawnto have the curved profile may intersect most if not all of thereceivers. As will be discussed later, some of the detectors may not lieexactly on the curved profile and may deviate from the curved profilewithin an acceptable range. In another exemplary embodiment, the profileof the curve on which the plurality of detectors is distributed isparameterized as will be discussed later in more details. According tostill another exemplary embodiment, a ramp rate of the distribution ofthe detectors varies along the streamer.

According to an exemplary embodiment, a streamer having part of thedetectors provided on a curved profile is shown in FIG. 5. FIG. 5 showsa system 50 having at least one streamer 52. The streamer 52 includes abody (e.g., cable) on which at least one detector 54 is provided. Forsimplicity, both the streamer and body are identified by the samereference number 52. However, as discussed with regard to FIG. 5, thestreamer may include both the body and the detectors. In oneapplication, the streamer includes plural detectors 54. The streamer 52may be connected to a vessel 56 for being towed under the water surface58. Towing the streamer 52 under water is different from lying downocean bottom cables at least for the following reason. The distributionof some of the detectors on a curved profile as discussed in theprevious embodiments is maintained substantially unchanged while towingthe streamer contrary to the situation when lying down cables anddetectors. In this last situation, the operator of the streamer is notinterested to use a certain profile for the detectors' distribution orto maintain that profile.

In one exemplary embodiment, a distance between consecutive detectors 54is constant. However, in another application, a distance betweenconsecutive detectors 54 varies. For example, the distance betweenconsecutive detectors at the beginning and end of the body may besmaller than a distance between consecutive detectors in the middle ofthe streamer. A distance between two consecutive detectors may be in therange of meters or tens of meters. A length of the streamer may be inthe order of kilometers.

In order to achieve the curved profile shown in FIG. 5, plural birds 60(or equivalents, like deflectors, etc.) may be provided along thestreamer. A bird may be simply a weight that sinks a corresponding partof the streamer to a desired depth assuming that the body floats innormal conditions. Alternatively, the bird may be a more or lesssophisticated device configured to maintain a desired depth under wateras well as separation between streamers in applications using multiplestreamers. For example, the bird may have wings or other devices formaneuvering up, down, left, right, etc. The bird, similar to thedetectors, may be electrically connected to the vessel for controland/or data collection. In another embodiment, the birds are selfpowered with the use of individual batteries.

A detector is a generic name for any device that is capable of measuringa pressure wave or another quantity (e.g., particle velocity ordisplacement of the medium) indicating the presence of an acoustic wave.An example of such a detector is a geophone or a hydrophone or anaccelerometer and they are known in the art. Thus, a detaileddescription of these devices is not provided herein.

The curved shape of the streamer 52 may have different profiles asdiscussed next. It is noted that the curved shape of the streamer isachieved while towing the streamer underwater. In other words, thiscurved shape of the streamer should not be confused with a situationwhen plural detectors connected to a cable (ocean bottom cable) isdeployed on the bottom of the ocean for passive measurements as notedabove.

In one exemplary embodiment illustrated in FIG. 6, the curved profile ofthe streamer 52 may be defined by three parametric quantities, z₀, s₀and h_(c). It is noted that not the entire streamer has to have thecurved profile. In other words, the curved profile should not beconstrued to always apply to the entire length of the streamer. Whilethis situation is possible, the exemplary embodiments do not prohibithaving the curved profile applied to only a portion of the streamer. Inother words, the streamer may have (i) only a curved profile or (ii) aportion having the curved profile and a portion having a flat profile,the two portions being attached to each other.

The first parameter z₀ indicates the depth of a first detector 54 a ofthe streamer relative to the surface 58 of the water. This parameter mayhave a value in the range of meters to tens of meters. For example, z₀may be around 6 m. However, as would be recognized by those skilled inthe art, the value of z₀ depends on each application and may be relatedto the depth of the bottom of the ocean, the depth of the reflectors,the power of the sound source, the length of the streamer, etc.

The second parameter s₀ is related to the slope of the initial part ofthe streamer 52 relative to a horizontal line 64. If the parameter s₀ isthe slope, as illustrated in FIG. 6, it is related to an angle made by atangent T to an initial part of the streamer and the horizontal line 64.It is noted that the slope of the curved profile at point 54 a is givenby a ratio of the change of the curved profile along the Z axis withrespect to the change along the X axis. The slope is thus equal to themathematical value of the tangent of the angle, i.e., slope (at point 54a in FIG. 6)=tan (of corresponding angle). Further, it is noted that forsmall angles (e.g., five or less degrees), tan (angle) is approximatelyequal to s₀. Thus, for small angles, the slope and the angle may be usedinterchangeably. In one embodiment, the value of s₀ may be between 0 and6 percent. The example shown in FIG. 6 has an initial slope s₀ equal tosubstantially 3 percent. It is noted that the profile of the streamer 52in FIG. 6 is not drawn to scale as a slope of 3 percent is a relativelysmall quantity. In one application, the slope may be constant for acertain length of the streamer and then it can change until approachingzero.

The third parameter h_(c) indicates a horizontal length (distance alongthe X axis in FIG. 6 measured from the first detector 54 a of the curvedportion of the streamer until an end of the curved portion. Thisparameter may be in the range of hundreds to thousands of meters. Forexample, h_(c) is around 3000 m for the configuration shown in FIG. 6.This parameter defines the end of the curved part of the streamer 52. Inother words, the streamer 52 may have a first portion 52 a that has afirst curved profile and a second portion 52 b that is either flat orhas a different curved profile. Parameter h_(c) defines the firstportion 52 a. It is noted that in one application the streamer 52 hasboth the first portion 52 a and the second portion 52 b while in anotherapplication the streamer 52 has only the first portion 52 a. Such anembodiment is illustrated in FIG. 7 in which the detectors 54 lie onlyon the first portion 52 a, the detector 54 b being the last detector onthe streamer 52. It is noted that a projection along line 64 of thelength of the streamer 52 in FIG. 7 may be less than h_(c). In otherwords, in some embodiments, the streamer does not extend along theentire curved profile, i.e., a length of the streamer projected on Xaxis is less than h_(c).

According to another exemplary embodiment, the curved profile of thestreamer 52 may be described, approximately, by the following equations:

$\begin{matrix}{{{z(h)} = {{z_{0} + {s_{0}{h( {1 - {0.5( \frac{h}{h_{c}} )}} )}\mspace{14mu}{for}\mspace{14mu} h}} \leq h_{c}}},{and}} & (1) \\{{z(h)} = {{z_{0} + {{s_{0} \cdot 0.5 \cdot h_{c}}\mspace{14mu}{for}\mspace{14mu} h}} > {h_{c}.}}} & (2)\end{matrix}$

In these equations, z is measured along the Z axis and h is measuredalong the X axis, where Z is perpendicular to the surface of the waterand X extends, along the surface of the water. Also, it is noted that insome applications, only equation (1) may be used to define the profileof the streamer, depending on the length of the streamer. In otherwords, in some embodiments, the streamer does not have to have a flatportion or other portions at the end of the curved portion. For seismicdata acquired with streamers disposed along a profile given by thesespecific equations, it was found that the clarity of the processedimages of the subsurface improved substantially, as shown in FIGS. 8 and9. It is noted that FIG. 8 illustrates the final image of the subsurfaceobtained with traditional acquisition devices while FIG. 9 illustratesthe final image of the same subsurface using the novel acquisitiondevice described above. Further, it is noted that the first equation (1)provides the curved profile while the second equation (2) provides astraight line (constant depth).

Those skilled in the art would understand that the values provided byequations (1) and (2) are approximate as the detectors are underconstant motion exerted by various water currents and the movement ofthe vessel. In other words, it is understood that detectors providedsubstantially on the curved profile described by equation (1) and/or(2), e.g., at positions as close as 10 to 20% to the real curve in termsof the actual depth z(h), are envisioned to be covered by the abovementioned equations.

In another exemplary embodiment, z(h) may be generally described by aparabola, a hyperbola, a circle, or any other curved line. In oneexemplary embodiment, the curved profile may be formed by two or moredifferent curved profiles, e.g., a parabola combined with a circle, etc.

In another exemplary embodiment, the birds 60 that are attached to thestreamer 52 are placed at specific locations so that the streamer takesthe desired curved profile, e.g., parabola, hyperbola, circle, etc. Forexample, if the circle is the profile of the curved streamer, a radiusof curvature may be around 50 km. Thus, according to this exemplaryembodiment, the detectors may not be exactly located on the desiredcurved profile but substantially thereon, e.g., in the range of 10 to20% of the actual depth z(h).

According to an exemplary embodiment, the depth of the birds and/or thedetectors may be between 5 and 50 m. However, those skilled in the artwould understand that, as detector technology improves, these ranges mayincrease to over 250 m. Thus, the numbers presented herein are forexemplary purposes and not intended to limit the applicability of theexemplary embodiments.

According to an exemplary embodiment, the curved profile of the streamerdiscussed above may be described in terms of a local ramp rate along thestreamer. For example, consider corresponding slopes at various pointsalong the streamer. If the change in the streamer's local slope (theramp rate) is plotted as a function of a horizontal position along thelength of the streamer on the X axis, as shown in FIG. 10, the differentcurves 70 to 76 describing the change of the ramp rate are characterizedby the fact that they have at least two different ramp-rate values. Thisis in contrast to curve 78 that describes a slanted streamer having aconstant slope (e.g., a slope that does not vary with the horizontalposition of the detectors along the streamer). It is noted that the ramprate may also increase along the X axis in FIG. 10.

According to another exemplary embodiment, a streamer having a firstregion with a slanted configuration and a second region having adifferent slanted configuration is illustrated in FIG. 11. Suchconfiguration allows sufficient notch dynamics to be obtained forshallow reflectors by using detectors lying at higher depths. Theconfiguration shown in FIG. 11 has the first region 52 a slanted at apredetermined angle and the second region 52 b substantially horizontal.

In the embodiment shown in FIG. 11, the seismic receiver closest to aseismic source 80 lies at a depth of substantially 7.5 m. The firstportion 52 a may have a length of between 1 and 3 km, for example 2 km.The seismic receiver 54 of the first portion 52 a that is farthest fromthe seismic source 80 lies at a depth of substantially 37.5 m. Thesecond portion 52 b being horizontal, the receivers 54 of the secondportion 52 b farthest from the source 80 also lie at a depth of about37.5 meters. These values are given by way of illustration. The depthcan be chosen in each case in relation to particular conditions (depthof the water section, geological characteristics, etc.).

FIGS. 12 and 13 illustrate some of the advantages of acquiring data witha novel streamer having a slanted first portion and a horizontal secondportion. The effective spectra 92 and 96 shown in FIGS. 12 and 13correspond to simulations including the ghosts for a shallow reflectorsimilar to that discussed with regard to FIGS. 3 and 4. The spectrumsimulations are for a streamer having the first detector placed relativeto the surface of the water at a depth of about 7.5 m and about 15 m. Inboth of these simulations, the last detector on the slanted streamer isplaced at a depth of about 37.5 m relative to the water surface. Thesimulated spectra (curves 90 and 94) neglect the ghost effect, i.e.,they represent an “idealized” situation in which the presence of ghostshas been artificially removed from the simulation. As clearly shown, thecalculated spectra 92 and 96 have shapes close to the effective spectra90 and 94, respectively, when the low frequency and high frequencybehaviour is considered, a result not obtained using the conventionalapproach shown in FIGS. 3 and 4.

By comparing the ideal spectra and the effective spectra of FIGS. 12 and13, it is observed that ghost elimination has been achieved, and thenotch at 45 to 50 Hz is suitably filled, which again is an improvementcompared to FIGS. 3 and 4.

According to an exemplary embodiment, the slanted first portion 52 a hasa slope of less than 2%. This slope avoids setting up turbulences whichwould be detrimental to the quality of the signals recorded by theseismic receivers. This slope is more preferably than 1% which providesa sufficient range of receiver depths to achieve good quality ghostelimination when processing.

According to another exemplary embodiment illustrated in FIG. 14, thedepth control birds 60 are adjusted so that the streamer 52 includesplural portions having different slopes. For example, FIG. 14 shows thestreamer 52 having a first region 52 a having a first slope, a secondportion 52 b having a second slope, a third region 52 c having a thirdslope and a fourth portion 52 d having a fourth slope. More or lessportions may be employed and the slopes of these portions may bepositive, negative, or a combination thereof. According to one variantof this embodiment, section 52 d may be horizontal, i.e., a sectionhaving substantially no slant. According to another variant, differentportions may form a single portion having a uniform slant.

The streamers may be solid streamers marketed by Sercel (CarquefouCedex, France) under the trademark Sentinel, but the exemplaryembodiments disclosed herein are applicable to other types of streamers.The depth control birds may be devices of Nautilus type (trademarkregistered by Sercel) which permit lateral positioning of the streamers,but other types of depth control devices can be used to implement theexemplary embodiments. The distances between adjacent receivers 54 areon the order of a few meters, and appropriately the distances betweenadjacent depth control birds may be between 200 and 400 meters.

It is noted that various combinations of the above-discussed embodimentsare also envisioned to be novel and possible to be realized, e.g., astreamer with a combination of curved profile portions and multipleslanted portions.

According to an exemplary embodiment illustrated in FIG. 15, there is amethod for seismic data acquisition related to a submerged subsurface.The method includes a step 1500 of towing a body having a predeterminedlength together with plural detectors provided along the body; a step1502 of configuring plural birds provided along the body to float at apredetermined depth from a surface of the water such that a firstportion of the body has a curved profile while being towed underwater;and a step 1504 of recording with the plural detectors the seismic datagenerated in the body of the water while towing the body. The curvedprofile may stay substantially unchanged while the vessel is towing thestreamer due to the birds.

According to another exemplary embodiment illustrated in FIG. 16, thereis a method for deploying a streamer for seismic data acquisitionrelated to a submerged subsurface. The method includes a step 1600 ofreleasing into the body of water, from a vessel, a body having apredetermined length together with plural detectors provided along thebody; a step 1602 of towing the body and the plural detectors such thatthe plural detectors are submerged; and a step 1604 of configuringplural birds provided along the body to float at a predetermined depthfrom a surface of the water such that a first portion of the body has acurved profile while being towed underwater.

The curved profile may be a parabola, a circle or a hyperbola, a secondportion of the streamer may have a flat profile and is connected to thefirst portion, and the curved profile may be described by z(h), which isa depth of a point of the first portion relative to the surface of thewater, and satisfies the following equation:

${{z(h)} = {{z_{0} + {s_{0}{h( {1 - {0.5( \frac{h}{h_{c}} )}} )}\mspace{14mu}{for}\mspace{14mu} h}} \leq h_{c}}},$

where h is a horizontal distance between the point and a first detectorprovided on the body, z₀ is a depth of the first detector relative tothe surface of the water, s₀ is a slope of the body at the firstdetector of the streamer, and h_(c) is a predetermined horizontaldistance between the first detector and an end of the curved profile.

The method may include a step of towing a second portion, connected tothe first portion, the second portion being described by equationz(h)=z₀+s₀·0.5·h_(c) for h>h_(c). A projection of the entire body on aline substantially parallel to the surface of the water may be less thanhc. In one application, the curved profile stays substantially the sameduring the towing.

The method may also include a step of towing a second portion, connectedto the first portion, the second portion having birds and detectors andhaving a flat profile, wherein the second portion is substantiallyparallel to the surface of the water. In one application, no portion ofthe body, detectors or birds are anchored to the subsurface.

In one exemplary embodiment, there is a streamer for collecting seismicdata related to a subsurface of a volume of water. The streamer includesa body having a predetermined length; plural detectors provided alongthe body; and plural birds provided along the body, wherein the streameris configured to flow underwater when towed such that the pluraldetectors are distributed along a curved profile described by (i) adepth z₀ of a first detector, (ii) a slope s₀ of a first portion of thebody with an axis parallel with a surface of the volume of water, and(iii) a predetermined horizontal distance h_(c) between the firstdetector and an end of the curved profile.

In another exemplary embodiment, there is a streamer for collectingseismic data related to a subsurface of a body of water. The streamerincludes a body having a predetermined length; plural detectors providedalong the body; and plural birds provided along the body. The streameris configured to flow underwater when towed such that the pluraldetectors are distributed along a first portion and a second portion ofthe body. The first portion has a slanted profile with a first slope andthe second portion has a slanted profile with a second slope differentfrom the first slope. In one application, the second slope is smallerthan the first slope. In another application the second slope issubstantially zero.

According to another exemplary embodiment, there is a streamer forcollecting seismic data related to a subsurface of a body of water. Thestreamer includes a body having a predetermined length; plural detectorsprovided along the body; and plural birds provided along the body. Thestreamer is configured to flow underwater when towed such that theplural detectors are distributed along a first portion of the body, andthe first portion has a slanted profile with a first detector configuredto float deeper than a last detector of the first portion.

According to still another exemplary embodiment, there is a streamer forcollecting seismic data related to a subsurface of a body of water. Thestreamer includes a body having a predetermined length; plural detectorsprovided along the body; and plural birds provided along the body. Thestreamer is configured to flow underwater when towed such that theplural detectors are distributed along a curved profile, wherein thecurved profile has an initial slope larger than a slope at the end ofthe curved portion.

The disclosed exemplary embodiments provide a streamer and a method forseismic data acquisition. It should be understood that this descriptionis not intended to limit the invention. On the contrary, the exemplaryembodiments are intended to cover alternatives, modifications andequivalents, which are included in the spirit and scope of the inventionas defined by the appended claims. Further, in the detailed descriptionof the exemplary embodiments, numerous specific details are set forth inorder to provide a comprehensive understanding of the claimed invention.However, one skilled in the art would understand that variousembodiments may be practiced without such specific details.

Although the features and elements of the present exemplary embodimentsare described in the embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the embodiments or in various combinations with or withoutother features and elements disclosed herein.

This written description uses examples of the subject matter disclosedto enable any person skilled in the art to practice the same, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the subject matter is defined by theclaims, and may include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims.

What is claimed is:
 1. A streamer for collecting seismic data related toa subsurface of a body of water, the streamer comprising: a body havingfirst and second portions; plural detectors provided along the body; andplural birds distributed along the first portion of the body, whereinthe plural birds are configured to position the body, when deployedunder water, at predetermined depths from a surface of the water toimpart to the first portion of the body a predetermined, variable-depthprofile while being towed underwater.
 2. The streamer of claim 1,wherein the variable-depth profile is one of a parabola, a circle or ahyperbola.
 3. The streamer of claim 1, wherein the plural birds are alsodistributed along the second portion to achieve a flat profile of thesecond portion.
 4. The streamer of claim 1, wherein the non-linear,variable-depth profile is parameterized and described by z(h), which isa depth of a point of the first portion relative to the surface of thewater, and satisfies the following equation:${{z(h)} = {{z_{0} + {s_{0}{h( {1 - {0.5( \frac{h}{h_{c}} )}} )}\mspace{14mu}{for}\mspace{14mu} h}} \leq h_{c}}},$where h is a horizontal distance between the point and a first detectorprovided on the body, z₀ is a depth of the first detector relative tothe surface of the water, s₀ is a non-zero slope of the body at thefirst detector of the streamer, and h_(c) is a predetermined horizontaldistance between the first detector and an end of the non-linear,variable-depth profile.
 5. The streamer of claim 4, wherein the secondportion is described by equation z(h)=z₀+s₀·0.5·h_(c) for h>h_(c). 6.The streamer of claim 4, wherein a projection of the entire body on aline substantially parallel to the surface of the water is less thanh_(c).
 7. The streamer of claim 1, wherein the non-linear,variable-depth profile stays substantially the same during the towing.8. The streamer of claim 1, wherein the plural detectors includehydrophones and accelerometers.
 9. A method for seismic data acquisitionrelated to a subsurface of a body of water, the method comprising:towing a body having plural detectors distributed along the body;configuring plural birds along a first portion of the body, to positionthe body at predetermined depths from a surface of the water to impartto the first portion of the body a predetermined, variable-depth profilewhile being towed underwater; and recording with the plural detectorsthe seismic data while towing the body.
 10. The method of claim 9,wherein the profile is a parabola, a circle or a hyperbola.
 11. Themethod of claim 9, wherein a second portion of the streamer has a flatprofile and is connected to the first portion.
 12. The method of claim9, wherein the non-linear, parameterized, variable-depth profile isdescribed by z(h), which is a depth of a point of the first portionrelative to the surface of the water, and satisfies the followingequation:${{z(h)} = {{z_{0} + {s_{0}{h( {1 - {0.5( \frac{h}{h_{c}} )}} )}\mspace{14mu}{for}\mspace{14mu} h}} \leq h_{c}}},$where h is a horizontal distance between the point and a first detectorprovided on the body, z₀ is a depth of the first detector relative tothe surface of the water, s₀ is a non-zero slope of the body at thefirst detector of the streamer, and h_(c) is a predetermined horizontaldistance between the first detector and an end of the non-linear,variable-depth profile.
 13. The method of claim 12, further comprising:towing a second portion, connected to the first portion, the secondportion being described by equation z(h)=z₀+s₀·0.5·h_(c) for h>h_(c).14. The method of claim 9, wherein the plural detectors includehydrophones and accelerometers.
 15. A method for processing seismic dataacquired with a variable-depth profile streamer, the method comprising:towing a body having plural seismic detectors; configuring plural birdsalong a first portion of the body, to position the body at predetermineddepths from a surface of the water, to impart to the first portion ofthe body a predetermined, variable-depth profile while being towedunderwater; recording with the plural seismic detectors seismic datagenerated by a seismic source while towing the body; and processing theseismic data to generate an image of a surveyed subsurface.
 16. Themethod of claim 15, wherein the variable-depth profile is one of aparabola, a circle or a hyperbola.
 17. The method of claim 15, whereinthe plural birds are also distributed along a second portion of the bodyto achieve a flat profile for the second portion.
 18. The method ofclaim 15, wherein the non-linear, variable-depth profile isparameterized and described by z(h), which is a depth of a point of thefirst portion relative to the surface of the water, and satisfies thefollowing equation:${{z(h)} = {{z_{0} + {s_{0}{h( {1 - {0.5( \frac{h}{h_{c}} )}} )}\mspace{14mu}{for}\mspace{14mu} h}} \leq h_{c}}},$where h is a horizontal distance between the point and a first detectorprovided on the body, z₀ is a depth of the first detector relative tothe surface of the water, s₀ is a non-zero slope of the body at thefirst detector of the streamer, and h_(c) is a predetermined horizontaldistance between the first detector and an end of the non-linear,variable-depth profile.
 19. The method of claim 18, wherein a secondportion attached to the first portion is described by equationz(h)=z₀+s₀·0.5·h_(c) for h>h_(c).
 20. The method of claim 15, whereinthe plural detectors include hydrophones and accelerometers.